In semiconductor manufacture, a single semiconductor die (or chip) is typically mounted within a sealed package. In general, the package protects the die from damage (e.g. breakage, physical abuse) and from contaminants in the surrounding environment. In addition, the package provides a substantial lead system for connecting the electrical devices formed on the die to a printed circuit board or other external circuitry.
Typically, the initial component in the packaging process is a leadframe or leadframe strip. A leadframe strip is a metal frame which is several inches long. The leadframe strip supports each semiconductor die for packaging and provides the leads for the final semiconductor package. A typical leadframe strip is produced from metal sheet stock (e.g. a copper alloy) and is adapted to mount several (e.g. eight) semiconductor packages. A leadframe strip typically includes side rails on either side which are formed with a row of circular openings or holes. The side rails and the holes in the side rails, facilitate transport and indexing of the leadframe strip by automated packaging machinery. As an example, the leadframe strips may be removably clipped to carrier belts that move the leadframe strips through the various manufacturing operations.
During a conventional packaging process, the dies are attached to mounting paddles formed on the leadframe strip. Also during the packaging process, the bond pads formed on each die are electrically connected to the lead fingers of the leadframe strip using fine bond wires. Following the application of a polymide protective layer to the faces of the dice, each die and a portion of the leadframe strip to which it is attached, is encapsulated in a plastic material to form the semiconductor. Following encapsulation, the semiconductor packages require a deflashing process for removing excess molding material from the package exterior. A trim and form operation is then used to separate the resultant interconnected packages and to bend the leads of each semiconductor package into the proper configuration.
During the packaging process it is often necessary to track the location of the dies or of the semiconductor packages, on the moving leadframe strips. As an example, following encapsulation of a die, it may be necessary to track the location of the semiconductor packages on the leadframes in order to imprint a visual code on the exterior of each semiconductor package. Typical information coded on the package includes product the, product specifications and the date of manufacture.
For imprinting such a visual code, a laser inscription process can be used. With laser inscription, a laser is pulsed in a predetermined pattern onto the semiconductor packages as the leadframe strip is moved by a carrier belt past the laser. The laser is typically fired by an optic sensing device that is triggered by the holes in the siderails, or other reference locations, on the leadframe strips. The optic sensing device includes a light source (i.e. sender) and a photocell (i.e. receiver) facing each other on opposite sides of the leadframe. As a reference location on the leadframe strips moves past the light source, the light beam from the light source passes through the reference location to the photocell to make (or break) a control circuit and fire the laser.
A problem with triggering a laser in this manner is that the reference locations must be precisely aligned with the path of the light beam (i.e. optic sight line) from the light source. Any misalignment between the light source and the moving reference locations will block the path of the light beam causing the inscription laser to misfire or to not fire at all. This may cause the printed visual code to be illegible or missing on some of the semiconductor packages.
It is sometimes difficult to align an optic sensor with reference locations on the leadframe strips, because the carrier belts for moving the leadframe strips are often very long (e.g. 200 feet) and move relatively fast (e.g. 30 ft/min). The carrier belts are thus prone to move up and down or oscillate in a wave like pattern with respect to a stationary sensing device. Since reference locations, such as the holes in the leadframe strips, are only about 5 mil in diameter, very little up and down movement of a carrier belt is required to completely misalign the light source and reference locations.
The present invention is directed to a tracking sensor fixture that is adapted to precisely align reference locations (e.g. holes) of a moving leadframe strip with the optic sight line of an optic sensor. The tracking sensor fixture can thus be used in a semiconductor manufacturing operation, such as during laser inscription, to facilitate timing of the process.
Accordingly, it is an object of the present invention to provide a novel and improved method and apparatus for mounting an optic sensor and for aligning reference locations on a moving semiconductor leadframe strip with the sensor. It is a further object of the present invention to provide a tracking sensing fixture for precisely aligning reference locations, such as reference holes, formed on a moving semiconductor leadframe strip, with the optic sight line of an optic sensor. It is yet another object of the present invention to provide a tracking sensor fixture for mounting optic sensors that is reliable, relatively inexpensive to construct and operate, and which is adaptable for use in large scale semiconductor manufacture.